1. Field of the Invention
The present invention relates to the field of materials handling and more particularly to the fields of blending a material and/or separating debris from a material. During blending, materials from different production batches are blended together to achieve a homogeneous material blend having desired properties. During debris separation, undesired debris is separated from conveyed materials prior to a desired processing operation conducted on those materials.
The invention has particular utility in producing blends of plastic pellets and/or debris-free plastic pellets used in the plastics extrusion industry.
2. Discussion Of The Prior Art
In the field of industrial materials processing, it is often desired to achieve a material which is blended from different sources to achieve a homogeneous blended material having desired characteristics. In the plastic pellet processing industry, for example, a blender is typically used in the final stages of preparing the plastic pellets for extrusion. Typically, the pellets are produced in batches and stored, with each batch having a particular chemical and/or physical property characteristic. Pellets of different batches, having these different characteristics, are blended together to achieve a homogeneous blended pellet batch which has desired end-user specified characteristics. For example, an end-user will typically specify the chemical and physical characteristics of the pellets which are desired for a particular extrusion operation, e.g., a desired polyethylene of a desired melt index, and the pellet supplier typically blends together polyethylene pellets of the various batches to obtain the user specification. As an example, one polyethylene pellet batch may have a melt index of +1.degree. over specification and another may have a melt index of -1.degree. below specification. By suitably blending the two polyethylene pellet batches together, the desired melt index is achieved.
During the process of manufacturing and conveying the plastic pellets, which are typically cylindrical in shape and of 1/8 to 3/16 inches in diameter and 1/8-3/16 inches long, plastic debris is created in the form of fines, streamers, "angle-hair" and "snake skins". This debris causes problems to an end-user, such as plugging of equipment, etc. and it is therefore desirable to remove this debris to the greatest extent possible from the pelletized material before it is sent to the end-user.
To better illustrate certain aspects of the invention, reference will first be made to FIGS. 1 and 2 which respectively illustrate two prior art material blenders. In a conventional bottom feed blender 11, illustrated in FIG. 1, a feed source 35 supplies plastic pellets to a conveying fluid stream generated by a blower 37. Typically, the blower 37 uses air as the conveying fluid and the conveying air stream conveys a stream of feed material from feed source 35 through conduit 33 and up a lift pipe 19 of the blender 11. The upwardly directed air stream and conveyed feed also pneumatically conveys material which is provided in the lower smaller diameter end 17 of blender 11 due to a physical spacing existing between the inlet end of lift pipe 19 and the conveying air stream inlet to section 17 of the blender. Thus, feed from feed source 35 as well as material within a lower portion of section 17 of blender 11 are conveyed upwardly through the lift pipe 19. At the top 23 of the blender 11, the conveyed feed exits an upper end 25 of the lift pipe 19 and falls by gravity onto a material inventory bed 27 provided within the interior walls 13 of blender 11. Blender 11 has a larger diameter upper section 13 to accommodate the inventory of feed material. The larger diameter upper section 13 and smaller diameter lower section 17 are interconnected by a funnel shaped intermediate section 15. The inventory 27 of feed material held within upper section 13 of blender 11 enters into vertical downcomers 29, which are spaced circumferentially around the interior periphery of the side wall of blender 11, through downcomer ports 21 which are spaced longitudinally along each of the downcomers 29. Feed material residing in the material feed inventory 27 enters the ports 21 and passes through the downcomers 29 to the bottom section 17 of the blender 11. Feed material in inventory bed 27 also moves downwardly to bottom section 17. The material in a lower end of bottom section 17 is pulled upwardly by the incoming upwardly directed conveying air flow.
After the blender 11 is loaded with sufficient incoming feed, the feed source 35 is cut off and the conveying air from blower 37 is used to continually recycle material within bed 27 of the blender 11. The blender then operates for a sufficient period of time to achieve a homogeneous blend of the material within the blender after which the blower 37 is shut off and the blended material is drained at drain valve 31 through the bottom of the blender 11. Thus, by the continual remixing and recirculation of material, such as plastic pellets, in the blender, a uniform blending is achieved.
One problem which occurs with the blender 11 of FIG. 1, is that too much pellet feed in the incoming air flow from blower 37, that is, a too large pellet concentration, will overload the air flow so that not enough pellets are pulled upwardly through the lift pipe 19 from the bottom section 17 of the blender. Thus, there is little or no pellet recycling and blending during the filling cycle of blender 11. Even if the pellet concentration from feed source 37 is adjusted properly, the recirculation ability of the blender is reduced during filling because the incoming air flow has the burden of conveying feed pellets from source 35 as well as recycling the pellets within the blender.
To overcome the problem with the blending apparatus illustrated in FIG. 1, one solution has been to provide separate blowers for conveying material into the blender and for recirculation of material within the blender. FIG. 2 illustrates such an arrangement in which blower 37 is used to convey through conduit 33 a feed from feed source 35. The feed is supplied to the top of the blender 11, which otherwise has the construction illustrated in FIG. 1. A separate recycling air flow is established by blower 43 which provides air in conduit 41 to the bottom section 17 of the blender as in the FIG. 1 arrangement.
Since blower 43 does not convey any feed, it only has the task of recirculating material within the blender 11 and thus is able to do a more efficient blending operation unhampered by the concentration of the feed source 35. Unfortunately, the approach illustrated in FIG. 2 has the disadvantage of requiring two separate blowers and associated conduits which increases the overall cost and complexity of the blending system.